1. Field of the Invention
The present invention relates to an optical network system and an optical coupling apparatus. The invention particularly relates to a technique suitable for use in connecting optical communication devices (modules), provided for servers and databases, as a full mesh via an optical path in an optical network system in which WDM (Wavelength Division Multiplex) technology is employed.
2. Description of the Related Art
Networks, such as LANs (Local Area Networks), which transmit electric signals therein have recently been built, and from the viewpoints of recent increasing network capacity (speeding up of transfer signals) and network expansion, networks, such as optical LANs, which transmit light signals therein have started to be constructed. Under such circumstances, WDM technology, in which multiple signals are multiplexed in a single communication path, or an optical fiber, without causing any crosstalk, has been regarded as an up-and-coming technology due to its large capacity, high extensibility, and high confidentiality.
(1) Examples of Optical Networks
(1.1) WDM Star Network
FIG. 9 shows an example of a previous optical network system employing WDM technology, which network system includes two or more servers and databases. Common types of network topology are a bus, star, and ring type, and the following description will be made of a network with a star topology.
In the star network of FIG. 9, light signals at different wavelengths of λ1 through λn (n is an integer greater than 2) are output from optical senders (E/O) 101 provided, one for each communication device such as a server, of the first section 100, and multiplexed by an N×M (N and M are integers greater than 2) optical coupler (star coupler) 200, and then split in power. The light signals pass through wavelength-selective optical filters 300, each of which selectively transmits a predetermined fixed wavelength, and are then received by optical receivers (O/E) 401 provided (connected), one for each communication device of the second section 400, for each wavelength λi (i=1 through n).
Further, though not shown in FIG. 9, the same goes for when light signals are sent from the second section 400 to the first section 100. Light signals are output from optical senders (E/O) provided, one for each server of the second section 400, and then multiplexed and split in the N×M optical coupler. After that, the signals pass through wavelength-selective optical filters, each of which selectively transmits a to-be-received wavelength, and are then received by optical receivers (O/E) provided, one for each communication device of the first section 100, for each wavelength.
An optical network with such a construction is disclosed also in the following patent document 5. Further, in the above example, wavelength selection is performed by the N×M optical coupler 200 and the wavelength-selective optical filters 300 in combination, and this function can also be implemented by the combination of an N×1 multiplexer 201 and a 1×M demultiplexer 202, both of which are wavelength-selective, as shown in FIG. 10. These N×1 multiplexer 201 and 1×M demultiplexer 202 are interconnected with each other via an optical fiber, with an optical amplifier 203 interposed therebetween, if necessary, depending on a transmission distance therebetween.
Since this arrangement simplifies a network construction, the extensibility of the network is increased, and it becomes easy to send the same signal to different destinations at the same time. However, partly since light sources provided, one for each optical sender 101, have fixed oscillation wavelengths, and partly since each wavelength-selective optical filter 300 on the receiver end has a fixed central transmission wavelength, it is impossible to change light signal paths after they have been determined once.
(1.2) Network Employing the Technology of Adding ID Information to Signals
A construction for realizing full-mesh connections in such an optical network has already been proposed. For example, as shown in FIG. 11, adding ID information to signals electrically realizes a full-mesh network.
In the network of FIG. 11, for example, alight signal output from an optical sender 101 of the #i-th port is multiplexed with light signals output from other optical senders 101 in the N×M optical coupler 200, and is then broadcast to two or more output ports. At that time, since the light signals contain ID information electrically added thereto, each optical receiver 401 is capable of recognizing whether or not to receive those signals, so that only a necessary signal can be received by the individual optical receivers 401. In addition, since signals simultaneously sent out from different input ports of the N×M optical coupler 200 enhance interference between signals, signal transmission is performed on the input ports with different timing.
(1.3) Network Including Variable-Wavelength Optical Receivers
WDM technology can also be used for realizing flexible path switching to establish full-mesh networks. FIG. 12 illustrates an example of an optical network including variable-wavelength optical receivers. In the optical network of FIG. 12, WDM signals at wavelengths of λ1 through λn are multiplexed and split by the N×M optical coupler 200, and output from each output port. Each variable-wavelength optical filter 301 is set so as to have a central transmission wavelength equal to the wavelength to be received by each optical receiver 401, thereby transmitting only a required signal which is then received by the optical receiver 401. Here, signal paths can be set in a flexible manner by changing the central transmission wavelength of the variable-wavelength optical filter 301. In this instance, use of such variable-wavelength filters on the receiver ends is proposed in the following patent document 4.
(1.4) Optical Network Including Variable-Wavelength Optical Senders
FIG. 13 shows an optical network including variable-wavelength optical senders. In the network of FIG. 13, each optical sender 102 has a variable-wavelength light source (LD) whose wavelength can be varied from λ1 to λn, and the oscillation wavelength of the light source is set at the same wavelength as that of its destination receiver. As a result, the light signal from each optical sender 102 is multiplexed and split in the N×M optical coupler 200, and then passes through each fixed-wavelength selective optical filter 300 to be received by the corresponding optical receiver (the aforementioned destination receiver) 401. Here, signal path setting can be performed in a flexible manner by changing the wavelength of the variable-wavelength light source provided for each optical sender 102.
(1.5) Network Including a Wavelength Converting Device
FIG. 14 shows an optical network including a wavelength converting device which converts light signals in wavelength, from an arbitrary wavelength into a desired wavelength. In the network of FIG. 14, alight signal sent from an optical sender 101 at a wavelength of λj is multiplexed, in the multiplexer 201, with light signals output from other optical senders 101, and then input to the wavelength converting device 500 to be converted into a wavelength to be received. The light signals having been converted into receive wavelengths are demultiplexed by a wavelength-selective demultiplexer 202 and then received by desired optical receivers 401. Here, signal paths can be set in a flexible manner by changing conversion wavelength settings of the wavelength converting device 500. However, because of non-linearity utilized in the wavelength conversion, realizing the wavelength converting device 500 requires a high level of technology in terms of conversion efficiency, conversion band, device size, and so on.
(1.6) Network Including an Optical Switch
FIG. 15 illustrates an optical network employing an optical switch. In the network of FIG. 15, light signals output from different ports (optical senders 101) are input to a (multiple ports)×(multiple ports) optical switch 600. The optical switch 600 includes MEMS (Micro Electro Mechanical Systems) mirrors prepared, one for each port, with which mirrors optical paths to optical receivers 401 are set to make desired optical receivers receive the corresponding signals. Here, signal paths can be determined in a flexible manner by changing the setting of the optical switch 600.
(1.7) Optical Network Including a Continuous Circulation Coupler
FIG. 16 shows an example of an optical network including a continuous circulation coupler. In the network of FIG. 16, input ports (input channels; 5 ports (channels) in this example) of continuous circulation coupler [AWG (Arrayed Waveguide Grating) router] 700 and their wavelengths are connected to output ports (output channels; 5 ports (channels) in this example) thereof and their wavelengths in a one-to-one relationship, in accordance with certain rules (continuous wavelength circulation). Five signals being multiplexed are input to the 5 input ports of the coupler 700 and also output from the 5 output ports thereof as 5 signals being multiplexed. Accordingly, this technique makes it possible to select network paths by changing the wavelengths input to the ports, so that an all-optical network can be realized (detailed in the following patent document 1).
Further, the following patent documents 2 and 6 propose an optical cross connect device formed by an optical coupler, variable-wavelength filters, and a wavelength converter. This technology makes it possible to switch multiple optical channels among multiple ports. Further, the following patent document 3 proposes an optical cross connect device which can perform signal switching for each channel or for each channel group, for use in a WDM network including an optical switch, coupler, and feedback loop.
[Patent Document 1] Japanese Patent Application Publication No. 2001-326610
[Patent Document 2] U.S. Pat. No. 5,694,499
[Patent Document 3] Japanese Patent Application Publication No. 2002-152784
[Patent Document 4] Japanese Patent Application Publication No. HEI 3-57333
[Patent Document 5] Japanese Patent Application Publication No. HEI 9-261243
[Patent Document 6] Published Japanese Translation of PCT International Publication for Patent Application No. HEI 2000-500314
However, electric signal-employed optical networks are disadvantageous in that the size of networks that can be established as a full-mesh is limited, from the viewpoints of crosstalk, reflection, power consumption, and so on. Further, if the optical networks shown in the previous technologies are formed of a light source with a fixed oscillation wavelength and wavelength-selective optical filters, each having a fixed central transmission wavelength, the networks can be realized at low cost. However, in that case, path settings (optical link settings) are uniquely determined, so that realizing a full-mesh construction resultantly becomes difficult.
Further, even if full-mesh networks are realized by the foregoing previous technologies, there still are problems as follows.
(a) Adding Electric ID Information to Signals (See FIG. 11):
In this case, time-based exclusive processing is required, so that signals can only be sent one by one. Thus, with an increased number of ports, the processing becomes very time-consuming. In addition, as the ID information is added electrically, load on electric circuitry will be increased.
(b) Applying Variable-Wavelength Optical Receivers and Optical Senders (See FIG. 12 and FIG. 13):
Variable-wavelength optical receivers and optical senders need to be provided for all the input and output ports, so the networks becomes expensive.
(c) Applying a Wavelength Converting Device (See FIG. 14):
As has already been described, non-linearity is utilized in wavelength conversion, so that a high level of technology is required to realize a good conversion efficiency, conversion band, and device size, thereby increasing costs and network size.
(d) Applying an Optical Switch (See FIG. 15):
When using MEMS mirrors, mechanical control, namely, mirror angle control, needs to be performed, so that high-speed switching is technically difficult. In addition, the technique cannot cope with an increased number of ports.
(e) Applying a Continuous Circulation Coupler (FIG. 16):
Since variable-wavelength optical senders or a switch for switching input and output ports is required, the network becomes greatly expensive.
In view of these, by using previous technologies including the above patent documents 1 through 6, it is difficult to realize full-mesh optical networks with simple construction at reasonable cost.